Pain in the upper part of the neck and restriction of neck movements are common symptoms. The patient may give a history of an injury that flexed the head and neck (as when he is hit on the back of the head) as the precipitating factor. Weakness and spasticity of all limbs can follow the event of trauma. A range of motor and sensory deficits can occur. The sensory deficits are relatively less severe. Severe injuries to the cervicomedullary cord may produce respiratory paralysis, coma and even death. The tissues are more supple and stronger in children and traumatic dislocations are relatively uncommon. However, pre-existing muscle/ligament disorder can lead to atlantoaxial dislocation.

Radiographs showing the lateral views of the atlantoaxial region with the head and neck flexed and extended are always done. The atlantodental interval of less than 3 mm is generally considered within the normal range in infants and children in Group A and Group B. The atlantodental interval of more than 3 mm usually confirms the diagnosis. Dynamic CT scan with appropriate reconstruction images shows the dislocation clearly. MR showing signal alterations are important and frequently confirmatory evidence indicating compression of the cervicomedullary cord. MR and CT angiography can show the relationship of the vertebral artery to the facets of C2 and C1. Such information is crucial when lateral mass fixation techniques are employed for surgery. MRI provides useful information regarding the nature of soft tissue alterations. The status of bones in general and of facets in particular should be elaborately evaluated on investigations.

The treatment of patients with atlantoaxial instability is a surgical challenge, and achieving a successful outcome for these patients is gratifying. The complications of surgery, however, are potentially lethal. Various methods of fixation have been described and used successfully in the treatment of patients with atlantoaxial instability. The techniques of craniovertebral fixation evolved during the twentieth century as the anatomy and biomechanics of the craniovertebral region became clearer. The aim of surgery in general is to restore normal or the best possible alignment and to achieve stability of the atlantoaxial joint. The techniques of fixation for atlantoaxial dislocation can be divided into midline procedures that involve the fixation of arch of the atlas with the lamina of axis and lateral mass fixation procedures. Midline methods of fixation include sublaminar wire fixation and metal loop fixation. Metal implants and fixation methods provide an initial period of fixation of the region facilitating bony fusion. Bony fusion takes about 3 months, and the metal implants should be strong enough to hold the region for that period. It ultimately depends on bony fusion to provide stability to the region. Bone harvested from the patient’s own iliac crest has been identified by several authors to be superior to any other form of bone graft or artificial material [34]. Compromise on using the type of bone graft can lead to inadequate fusion, a phenomenon that can mar the entire operative procedure. Multiple pieces of corticocancellous bone graft should be placed in the craniovertebral region after elaborately preparing the host bone. The movements in the atlantoaxial region occur in atlantoaxial joint. Any method of stabilisation that directly fixes the joint and the fixation is segmental in nature, having superiority over methods that employ fixation remote to the site of maximum movements and those that attempt to fix multiple bone segments [29]. The lateral mass fixation techniques have been identified to be philosophically superior and biomechanically stronger to the techniques that involve fixation of the midline structures like lamina and arch of atlas (Fig. 4). In children with multiple joint ligamental laxity, occipitocervical fixation may sometimes be necessary. Methods incorporating screw fixation of the occipital end of the implant and inclusion of C1 and C2 facets by screw fixation were first described by us [14] (Fig. 5).

Several theories have been suggested to elucidate the probable cause and origin of basilar invagination. Most of these theories point towards embryological dysgenesis, genetic abnormalities and viral infections [5, 26, 36, 55, 56]. Several authors, for over a century, have thought that deformation had a mechanical cause and therefore applied the name ‘impressio baseos cranii’ (Berg and Retzius is cited by Virchow 1857) [55], or basilar impression [5]. Grawitz [36] believed that basilar invagination was often a result of under- or maldevelopment of the craniovertebral transition region. A range of malformations is frequently associated with anomalies of the atlas and axis, some of which may be quite atypical, and with fissures or defects and bone projections from the spinal column in the craniovertebral transition zone. The latter anomalies were grouped together by von Torklus and Gehle as suboccipital dysplasias [56].

Basilar invagination can be secondary to abnormally inclined alignment of the facets of atlas and axis [16, 19, 42]. The progressive slippage of the atlas over the axis secondary to this malalignment, a process similar to spondylolisthesis in the lumbosacral spine, results in invagination of the odontoid process into the craniocervical cord [42]. Short neck, low hairline, web-shaped neck muscles, torticollis, reduction in the range of neck movements and several such physical variations have been described as hallmarks of basilar invagination for over a century. A number of bone fusion deformities and platybasia have also been recorded. Neck pain, muscle spasms and restriction of neck movements are frequently noted and suggest instability of the region

In the year 1997, we presented a classification system for basilar invagination that divided it into two discrete categories. This classification helped in clarifying the understanding of the pathology and pathogenesis of the anomaly, in the selection of the surgical treatment and in prediction of the outcome [16]. Based on a single criterion of the absence or presence of Chiari malformation, the anomaly was classified into Groups I and II, respectively.

Essentially, Group I included cases where there was invagination of the odontoid process into the foramen magnum and it indented into the brainstem. The tip of the odontoid process distanced itself from the anterior arch of the atlas or the inferior aspect of the clivus. The distancing of the odontoid process from the anterior arch suggested the presence of instability of the region and atlantoaxial dislocation. The angle of the clivus and the posterior cranial fossa volume were essentially unaffected in these cases. In Group II, on the other hand, the assembly of odontoid process, anterior arch of the atlas and the clivus migrated superiorly in unison resulting in reduction of the posterior cranial fossa volume, which was the primary pathology in these cases. The Chiari malformation or herniation of the cerebellar tonsil was considered to be a result of reduction in the posterior cranial fossa volume. In the year 1997, Goel first defined the clinical implication of association of small posterior cranial fossa volume and Chiari malformation [16].

In our recent study, we identified a subgroup of patients where there was clear radiological evidence of instability of the region that was manifested by distancing of the odontoid process away from the anterior arch of the atlas and the radiological features matching those of Group I cases. Considering this current evaluation we have proposed a new classification for basilar invagination into two groups based on parameters that determined an alternative treatment strategy [19]. In Group A basilar invagination, there was a ‘fixed’ atlantoaxial dislocation, and the tip of the odontoid process ‘invaginated’ into the foramen magnum and was above the Chamberlain’s line [5], McRae’s line of foramen magnum [47] and Wackenheim’s clival line [54]. The definition of basilar invagination of prolapse of the cervical spine into the base of the skull, as suggested by von Torklus [56], was suitable for this group of patients. Group B basilar invagination was where the odontoid process and clivus remained anatomically aligned despite the presence of basilar invagination and other associated anomalies. In this group, the tip of the odontoid process was above the Chamberlain’s line but below the McRae’s and the Wackenheim’s lines. The radiological findings suggested that the odontoid process in Group A patients resulted in direct compression of the brainstem. Essentially, in Group A basilar invagination, the pathogenesis appeared to be mechanical instability of the region that was manifested by the tip of the odontoid process distancing itself from the anterior arch of the atlas or the lower end of the clivus. In some Group A patients, there was Chiari malformation, and this feature differentiates the present classification from the earlier classification. In this group, the atlantoaxial joints were ‘active’, and their orientation was oblique as shown in the figure, instead of the normally found horizontal orientation. Similarities of such a position of the C1–2 facets with spondylolisthesis seen in the subaxial spine can be seen. It appears that the atlantoaxial joint in such cases is in an abnormal position, and progressive worsening of the dislocation is probably secondary to increasing ‘slippage’ of the facets of atlas over the facets of axis. In Group B, the pathogenesis appeared to be congenital dysgenesis, and atlantoaxial joints were normally aligned or were entirely fused. The pathogenesis of basilar invagination appears to be different in the two groups. Understanding of these two types of basilar invagination is crucial in understanding the various involved management issues.

Majority of patients with Group A basilar invagination (58 %) had a history of minor to major head injury prior to the onset of the symptoms [19]. The pyramidal symptoms formed a dominant component. Kinaesthetic sensations were affected in 55 % cases. Spinothalamic dysfunction was less frequent (36 %). Neck pain as a major presenting symptom was in 77 % cases. Torticollis was present in 41 % cases. The analysis of radiological and clinical features suggests that the symptoms and signs were a result of brainstem compression by the odontoid process.

The presentation was relatively acute in Group A cases, whilst it was long-standing and slowly progressive in Group B cases. In Group B cases, the onset of symptoms and their evolution were insidious. Thus, a careful inquiry was needed to pinpoint the timing of appearance of the initial symptoms. In the majority of cases, the time of onset was already forgotten. This made it difficult to establish with certainty the age at which the complaints began. The symptoms and signs in Group B basilar invagination appeared to be directly related to the ‘crowding’ of neural structures at the foramen magnum. Although the dimensions of the foramen magnum were large and sometimes larger than even in a normal state, the volume of its contents and probably the ‘pulsatile’ compression of the structures at the foramen magnum resulted in neurological symptoms. The markedly reduced girth of the brainstem in Group A cases clearly showed that direct compression of the brainstem by the odontoid process caused the neurological symptoms. Central cord symptoms and related signs were noted in cases associated with syringomyelia.

Occipitalisation of the atlas: Occipitalisation of the atlas associated with basilar invagination was noted first by Rakitansky [cited by Grawitz [36]] and has since been referred to frequently [36, 47, 54, 55]. Many authors have regarded assimilation as a characteristic feature of basilar invagination. The assimilation of atlas can be partial or incomplete.

Klippel–Feil bone anomaly refers to the triad of observations of extensive cervical vertebral fusions, low hairline, restrictions of neck movements and a short neck.

Trauma of varying severity was a noteworthy precipitating factor in Group A cases [19]. Trauma seldom plays any major role in precipitating the symptoms in Group B cases. The fact that trauma influenced the acute development of symptoms pointed towards an element of instability of the craniovertebral region in Group A patients.

Age factor: Basilar invagination Group A is more often seen in younger patient group, whilst Group B basilar invagination is more frequently encountered in older patient group.